Flash bulb igniter



FLASH BULB IGNITER Filed 001'.. 7, 1946 Patented Oct. .25, 1949 UNITED sTATEs PATENT oi-Ficr-z 2,4sao1o y Applicationstlilll. (13142721: No. 701,849

This invention relates generally to photographic lighting devices andmore particularly to devices used to remotely control the igniting of a so-called ash bulb such as is used in making ilash light pictures.

Where the field to be covered by a camera is relatively small, it is a fairly simple matter to illuminate this field by two or more flash bulbs which may be located at different points, but which are nevertheless controlled by a single switch and source of power. However, when the illumination requirements are such that the field must be illuminated by flash bulbs located at widely separated points, the power requirements of the individual bulbs may be such that an excessive voltage drop will be experienced by each of the bulbs if all are energized by a single source of power. While all of the bulbs will normally be ignited, the voltage drop will have increased the heating time of the fuse or filament within the ilash bulb, and while this delay may be very slight, it' is nevertheless sumcient to destroy any synchronism between the various ash bulbs, and between indivdual bulbs and the shutter of the camera.

Various attempts have been made to insure the synchronization of the various flash bulbs with each other and with the camera shutter, but these attempts have generally been unsuccessful because of the voltage drop mentioned, or, where photo-electric controls have been used, by stray or random light falling on the photocell which has caused the premature discharge of the ash bulb. This dimculty has been particularly acute when exterior scenes are being photographed, since headlights of passing automobiles have caused t e premature discharge of the flash bulbs d` prevented the securing of l proper illumination for the picture.

It is therefore the major object of this invention to provide an igniter for flash bulbs which may be controlled from a distance and synchronized with other ash bulbs.

Another object of the invention is to provide such an igniter wherein the time delay between the master control and the ignition of the ilash bulb ds in the order of microseconds, so that synchronization problems are reduced to a minimum.

It is a further object of this invention to pro- 2 vide such an igniter which is controlled by the discharge of a master flash bulb, but which is not operated by'stray or random light of any norrial intensity-usually encountered in such wor A further object ofthe invention is to provide such an igniter which is of relatively light weight,

compact, and readily portable, and which requires no connection to any external electrical circuit.

It is a vstili further object of the invention tol provide such an igniter which is quite simple and may be used by relatively inexperienced operators, while at the same time being relatively inexpensive to manufacture.

These and other objects and advantages or the invention will become apparent from the following description of a preferred form thereof, and from the drawing illustrating that form in which the single figure is a schematic wiring diagram of the circuit used in this igniter.

The circuit includes a photo cell vIII which is adapted to trigger a thyratron II to cause a capacitor I 2 to discharge through a special transformer I3 and ignite a ash .bulb I 4.

Considering the circuit in somewhat greater detail, it will be seen that the anode I5 of photo cell I0 is connected by aconductor I8 to the positive terminal I1 of a source of power such as battery B. The cathode 20 of photo cell I 0 is connected, by conductor 2| to a terminal 22, and from the latter through a resistor 23 to a ground or common connection 24. The negative terminal 25 of the source of power previously mentioned is connected through conductors 26 and 2l! to the common connection 24; and the photo cell III will thus have the voltage of the power source impressed across its terminals, less any voltage drop across vresistor 23. When no light falls upon the photo cell III, the current through the latter will be relatively small and there will be substantially no voltage drop across resistor 23. However, when thel photo cell is illuminated, the current is increased and the voltage drop across resistor 23 is increased, changing the voltage of terminal 22 with respect to common connection 24. This voltage change is utilized as will hereinafter appear. l y

For reasons which will presently become apparent, the photo cell l 0 should be one which has 3 a sensitivity in the infra-red region oi' the spec trum. and should have a relatively high sensitivity with a relatively large current carrying capacity. 'I'his latter feature indicates that a gas-illled photo cell should be used, and it has been found that the cell commercially available under type 930 meets these requirements very' satisfactorily. Other models may, of course. be used, the model number here given being by way of example only and not as a limitation.

The thyratron is preferably of the indirectly heated cathode type, and includes an anode 30. a screen grid 3|. a control grid 32, a cathode 33. and a heater 34. The heater 34 has one of its terminals connected by conductor 35 to conductor 26. and its other terminal is connected by conductor 35 to one'of the terminals of a manually operated switch 31. The other terminal of switch 31. is connected bv conductor 40 to the negative terminal of 'a source of power such as a battery 4|. the positive terminal of which is connected by conductor 42 to conductor 25. Switch 31 thus controls the heating of the cathode 33: and since the high voltage source of power B may be permanently connected to terminals |1 and 25. this switch also controlsthe operation of the complete circuit.

Conductors 43 and 44 connect cathode 33 and screen grid 3I to grounded lead35.' A tube connected in this manner reouires a. relatively small change or swing in the voltage applied to its control grid in order to cause it vto conduct, while a gas-filled triode without the screen grid 3|, requires a much larger voltage swing if the tube is to be operated under stable condi-tions. As will be hereinafter pointed out it is desirablethat tube II have a relatively high current carrying capacity and for this reason an indirectly heatedv type cathode is preferable. While other tubes may be used in thiscircuit. for the reasons given above and for other reasons which will later'appear, a tube meeting allthese requirements -is very desirable, and such a. tube is commercially available under the type number GL-546.

To provide the requisite bias voltage for the control grid 32, a pair of resistors 45 and 4B are connected in series to form a voltage divider which is connected to conductors 26 and 40. Terminal 41, which is the common point of'connection of resistors 45 and 46, is thus at a. potential intermediate between conductors 26 and 40, and in the case illustrated. is at a negative potential with respect to conductor 26 and hence cathode 33. Control grid 32 is connected by conductors 50 and 5I to terminal 41, and by a. choice of suitable values for resistors 45 and 45. the control grid will be biased with a two-volt negative bias when the battery 4| is adapted to furnish six volts to the heater 34. This arrangement permits the bias to be automatically decreased in the proper ratio as battery 4I is discharged; and furthermore, the resistors 45 and 46 may be selected so that their resistance, in parallel, will be such `as to furnish the proper resistance from control grid 32 to common connection 24. To complete the circuit of the control grid, a coupling capacitor 52 is connected between Iterminal 22 and conductor 50 so that any voltage change appearing at terminal 22 will be transmitted to the control grid 32 to con-` trol the operation of the thyratron I I.

Transformer I3, described hereinafter in greater detail, is provided with a primary winding 53 and a secondary winding 54. One terminal of primary 53 is connected by conductor 55 to an-v ode 33 of tube II, and the Vother terminal is conof transformer I3; when designed to provide the nected by conductor 55, .terminal 51. conductor 5I, resistor and conductor BI to terminal I1. Capacitor I2 is connected .by conductors 32 and 53 to terminal 51 and conductor 26 respectively. Capacitor I2 is thus charged through resistor 60 by the source of power connected to terminals I1 and 25, and this same voltage is applied through primary winding 53 to anode 30 of tube II. The anode circuit of tube II may now be traced from cathode 33, to anode 33 and then through primary 53 to terminal 51, through conductor 58, resistor 5|) and conductor 5I, to terminal -|1.of source of power B, and from the latter through conductors 26 and 35 to conductor 43 and to cathode 33. An alternate circuit may be traced from terminal 51, through capacitor I2 to conductor 53 and then on, Ithrough conductor 35, as in the previous circuit.

The circuit for the photo cell I0 has already l been traced from terminal I1, through conductor l5, to anode l5, and then from cathode 20, through conductor 2| to terminal 22 and through resistor 23 to common connection 24 and then to terminal 25 of the source of power. As previously mentioned, a rising change of illumination on the photo cell I0 causes a positive voltage change at terminal 22, and this is applied to the input or grid circuito! thyratron II through capacitor 52, conductor 50 to control grid 32, and from there to cathode 33, through conductors 43, 35 and 21 to common connection 24 and back through resistor 23 to terminal 22. The ignition device is completed by connecting secondary 54 of' transformerl I3 to flash bulb I4 by conductors 64 and 55, one of which, if desirable, may be connected to the common connection 24.

One of the features of this invention is the use of the special transformer-I3 which is designed to pass a. pulse or transient, as opposed to operation with a sinusoidal or comparable type of alterimpedance of the thyratron II is much higher (e. g. 214 ohms). In order to realize the maximum heating of the lamp filament, maximum power must be drawn from the thyratron-as a generator; and to do this, the impedance of the load must match the impedance of the generator. Both of these problems may -be solved by the use correct voltage and current ratios, the transformer also offers the correct impedance match. Thus, assuming a typical voltage transformation ratio of 29 to 1, the impedance ratio is 29 or 840 ohms. 840 times .4 (the resistance of the ilash bulb), or 336 ohms, which is a sufilcient match with the 214 ohms internal impedance of the thyratron for the necessary power delivery to the flash bulb.

However, since transformer I3 is energized by a pulse. as will be later described, the characteristics of the transformer are different from the usual transformer adapted to be continuously energized by a sinusoidal or other steady state alternating current.

Neglecting resistances, which will normally be 'relatively minor factors in such a transformer,

theprimary winding 53 may be considered as being the equivalent oi' two inductances in series, one inductance Lx. being the leakage inductance oi' the primary with respect to the secondary, and the other inductance Lr being the primary inductance. The leakage inductance Lr. is the resuit of the magnetic flux of the primary which fails to link the secondary winding B4; while the primary inductance Li is the self-inductance of theprimary. Since the fuse or nlament of the flash bulb I4 acts substantially as a short circuit of the secondary winding 54, this short circuit resistance is reflected to the primary winding 53, and, in effect. short circuits the primary inductance L1 to leave only the leakage inductance Lx. to limit the rate of rise of current in the thyratron plate circuit. In order to secure the most efficient operation of the circuit, the leakage inductance should be kept to the minimum value which will limit the current in the primary to the allowable current rating of the thyratron, while the primary inductance, whose stored energy acts to maintain the'output after the initial steep wavefront, should be as high as possible. It has been found that the leakage inductance may vary between one-half and fty millihenries as limits but that best results are obtained when the value of Lr. is between 4 and 8 millihenries. Similarly, the value of the primary inductance may vary from 0.3 henry to several henries, there being no upper limit which shouldnot be exceeded, except those encountered in design which will cause the value of the leakage inductance to be raised above desirable values. The value of the primary inductance LP should be maintained relatively large since it, in eiect, holds up or maintains the secondary current after the initial steep rise caused by the firing of the thyratron Ii. This permits the effective use of the energy, stored in capacitor I2, and ignition of the ilash bulb I4 is insured. The pulse transformer I5 is thus somewhat dierent from the usual form of low frequency power transformers, since the pulse transformer is designed to have a minimum leakage inductance Lr., a maximum primary inductance Lp, and to be operated with its secondary winding 54 short circuited by the flash bulb I4. It will be appreciated that other factors enter into the computation and calculations of the windings 53 and 54, but these factors, such as distributed capacity, generally control the decay of the pulse in the secondary circuit, which decay occurs after the fuse or lament element of flash bulb I4 has been operated, and hence is of no concern in this device.

Operation When the flash bulb igniter herein described is to be used, it is placed in the proper position. to illuminate the scene to be photographed, and it is also located so that it will receive the direct light from the principal or master flash bulb which is normally located adjacent the camera. Swith'3l is closed so that battery 4I is connected to heater 34 to energize the latter and heat cathode 33, but the bias provided to control grid 32 by resistors 45 and 46 prevents any electrons emitted by the cathode from reaching anode 30 and triggering thyratron Il. The voltage source B connected to terminals I1 and 25, which is preferably one or more miniature batteries adapted to Aprovide approximately l90 volts, is connected through resistor 60 to capacitor I2 and charges the latter. The same voltage is applied to the photo cell I0, and a relatively small current,

and cause a voltage drop to appear between terminal 22 and common connection 24. However, since this illumination is relatively constant, the voltage appearing between these terminals will be relatively constant, and will have no 1 effect upon the control grid in its triggering of thyratron II. Since no current iiows between anode 30 and cathode 33' of thyratron II, primary 53 of transformer I3 is not energized and flash bulb I4 is not igmted.

However, when the control or master ilash bulb is ignited, generally by some synchronizing means connected with the shutter of the camera taking the picture, the light falling upon photo cell Il is greatly increased in a very short space of time. As a result, the current flowing through the photocell I0 is rapidly increased and the voltage difference between terminals 22 and 24 changes very suddenly. Resistors 23, 45, and 48 combine with capacitor 52 to form, in effect, a high pass filter which, when the light change is rapid enough, transmits the resulting change to superimpose the latter upon the normal bias of the control grid 32, causing the latter to swing toward a positive potential with respect to cathode 33 and thereby triggering the thyratron II. When this occurs, the charge which has been accumulated on capacitor I2 is permitted to flow through primary winding 53 of transformer I3, thereby providing a pulse which causes an induced current to flow in secondary Winding 54 to ignite the fuse or filament in iiash bulb I4. The values of capacitor I2 and resistor 60 are so chosen that the time required for the capacitor to charge is greater than the de-ionization time of thyratron Ii so that more than one discharge is avoided, and there is but a single pulse provided for each iiash of light falling on photo cell I0.

The source of power connected to terminals I7 and 25 provides a voltage sufficient to maintain the thyratron II in a conducting condition, but the current through resistor 60 causes a voltage drop which, when the condenser charge is dissipated, reduces the effective plate voltage of the thyratron below the value necessary to maintain ionization. The vtube then de-ionizes, as previously mentioned, and the grid regains control in less time than that required for condenser I2 to recharge through resistor 60.

To prevent the thyratron II from being triggered by a flash of relatively low intensitylight, or by a slow increase of light to a, great intensity, the values of capacitor 52 and of resistors-23, 45, and 46 are selected so that only voltage changes or impulses having a, relatively steep wave front are conducted through capacitor 52 without substantial attenuation or diminution, as previously mentioned. The characteristics of the thyratron Il are such that the control grid 32 must be made slightly more positive than the bias point in order'to trigger the tube, andto do this requires a sudden increase of light upon the photo cell, thus developing a transient voltage of steep wave front across resistance 23 which will be transmitted through .capacitor 52. The increase of light must be very fast .to provide a voltage of proper strength on the control grid, and this prevents the thyratron from ring when headlights, etc., are ilashed on the photo cell.

As previously mentioned, the photo cell is preferably sensitive to infra-red radiation and in this way it responds to the heating of the fuse or filament in the master flash bulb before th.

principal source of light in the latter, whether it is foil, wire, or other material, can be ignited to provide the principal source of illumination. As

a result, the thyratron Il is triggered and ash bulb Il is ignited in the shortest possible time after the circuit to the master flash bulb is closed. The delay inherent in this slave circuit does not exceed a few milliseconds and is generally in the order of microseconds. Furthermore, when the time of firing of flash bulb I4 is compared to the time of firing of the master flash bulb, the slave flash bulb is ignited at the same time or a few microseconds before the master bulb, thereby indicating that the triggering of thyratron Il is caused by the igniting of the fuse or filament of the master bulb instead of the principal flash following thereafter. However, the circuit conl nect a source of power to said capacitor to charge stants are such that a 100 watt bulb may be located three feet from the photo cell I0 and repeatedly turned on' and on without triggeringthyratron Il. The discrimination of the circuit is'thus clearly demonstrated.

While various' values may be assigned to the different components of the system, the following values, given by way oi example only and not as a limitation, have been found to produce excellent results:

The voltage of battery 4i, as previously mentioned, is 6 volts, while the voltage of the power source Bis 90 volts.

By using relatively small dry cell batteries, the

entire assembly may be housed in a relatively small, light weight cabinet which is readily port-` able and may be placed wherever necessary. All connecting wires necessary for synchronizing the master flash bulb with the camera may be run from the latter to the master unit; and with the various values given and with the transformer I3 constructed in accordance with the requirementsy previously stated, flash bulb I4 is ignited whenever the light from the master flash bulb falls upon photo cell I0. While, of course, it would be possible to separate these slave units at such a distance from the master flash bulb that the amount of illumination reaching the photo cell i0 would be insuflicient to trigger the thyratron Ii, such conditions normally do not occur in the photographing of a scene of any reasonable dimensions.

the latter; a discharge circuit for said capacitor including electronic switching means adapted to connect said capacitor to the primary of said pulse transformer whereby said capacitor may discharge through .said primary to produce a current pulse therein; and means for connecting the secondary of said pulse transformer to the fuse of said ilash bulb whereby the latter is ignited by a greater current pulse appearing in said secondaryk as a result ofv said pulse in said primary.

2. An igniter for a flash bulb of the type having an igniting fuse adapted to be melted by the heat of a firing current passed therethrough and containing combustible and combustion supporting materials, which includes: a photocell; a resistor connected to the cathode of said photocell; means adapted to connect said` photocell, in series with said resistor, to a source of electrical power; a gas-nlled electron discharge tube ofthe thyratron type having a cathode, an anode, and a control electrode; a coupling capacitor having one terminal connectedto said control electrode and the other terminal connected to said cathode of said photocell whereby a very rapid increase to `a great intensity of the light falling on said photocell will trigger said thyratron; meansfadapted to cause said cathode of said thyratron to emit electrons; meansnormally adapted to maintain the control electrode of said thyratron at a slight negative vpotential with respect to said cathode thereof; a step-down pulse transformer having a secondary winding adapted to be connected to a flash bulb, and having a primary winding; a power resistor; means connecting one terminal of said primary winding'to said anode of said thyratron,`and connecting the other terminal .of said primary winding to one terminal of said power resistor; means adapted to connect the remaining .terminal ofsaid power resistor to the It will thus be seen that a flash bulb ignition device has been provided which is reliable in operation, light in weight, and simple in operation. The voltage drain on the batteries is relatively small, and by using the voltage divider comprising the resistors 45 and .46, as the voltage of the battery 4| decreases, the bias on control grid l2 likewise decreases so that stable operation oi the system results so long as sunicient power remains in the battery to heat the cathode 33 to the proper operating temperature.

While a preferred form of the system has been shown, it is obvious that modifications may be made ytherein which in no way depart from the positive terminal of a source of power, and to .connect said cathode of said thyratron to the negative terminal of said source of power; and a discharge capacitor having one terminal connected to said negative terminal of said last' mentioned source of power', and the other terminal connected to the point of common connection of said primary winding and said power resistor, whereby said thyratron is normally non-conducting, but is triggered-as a result of a brilliant flash of light illuminating said photocell, whereupon said discharge capacitor is discharged and the pulse therefrom is delivered by said pulse transformer to the fuse of a-flash bulb to ignite the latter.

3. An igniter for a flash bulb of the type hav-j lng an lgnltlng'fuse adapted to be melted by the" heat of a ring current passed therethrough and containing combustible and combustion supporting materials, which includes: a capacitor; means for connecting a source of power to said capacitor to charge the same; a thyratron having cathode, anode, and control grid electrodes; a step-down pulse transformer having primary and secondary windings; ananode circuit for said thyratron ineiuding therein said capacitor' and-theprimar'y REFERENCES CITED of said transformer; a photocell; a, control grid circuit connecting said photoceil to said control lhf cggwlferens are of record m the grid,` a high pass filter in said control grid cirv p cuit preventing the triggering of'said thyratron v5 UNITED STATES PATENTS excepting by a high frequency transient Voltage; Number Name Date and means for connecting the secondary winding 2 071 370 Williams et al Feb 23 1937 of said transformer to the fuse of saidv flash bulb. 2272102 Wdman Fe'b '3 1942 2,595,600 Weisglass Feb. 26, 194s ALBERT K' EDGERTON- 2,399,222 Germeshausen Apr. 3o, 1946 2,408,764 Edgerton Oct. 8, 1946 

